1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary cell, and more particularly, to a high-voltage charge type non-aqueous electrolyte secondary cell.
2. Background Art
Mobile information terminals such as portable telephones, notebook personal computers, and PDAs are increasingly sophisticated and increasingly reduced in size and weight these days. These terminals use as their driving power sources a variety of non-aqueous electrolyte secondary cells typified by lithium ion secondary cells having a high-energy density and a high capacity. In recent years, these cells are expected to have a higher capacity to meet the demand for higher sophistication of these terminals. One technique known to achieve a higher cell capacity is to improve the utilization of positive electrode active material by charging the positive electrode until the potential of the positive electrode exceeds 4.3V.
As an example of a technique for improving the utilization positive electrode active material, Patent Document 1 below suggests using a positive electrode active material prepared by mixing a lithium cobalt oxide containing a different element and a layered lithium nickel manganese oxide.
In this technique, the lithium cobalt oxide has a different element such as zirconium (Zr) or magnesium (Mg) added thereto to improve its structure stability during charging at a higher potential than a positive electrode potential of 4.3V versus lithium. In addition, the lithium nickel manganese oxide has a layered structure so as to improve its thermal stability in a high potential region. A concurrent use of these two composite oxides improves the stability of a cell during high-voltage charging.
The above-described technique makes the positive electrode active material more resistant to damage during high-voltage charging higher than 4.3V, but has a problem of the oxidative decomposition of an electrolyte on the positive-electrode side during high-voltage charging. This results in a decrease in the high-temperature storage characteristics of the cell.
It is known that high-temperature storage characteristics can be improved by making the ethylene carbonate (EC) content of the non-aqueous solvent of the electrolyte less than 30% by volume, and possibly less than 25% by volume. Ethylene carbonate may cause degradation of electrolytes, and therefore, limiting its content prevents the degradation of the electrolyte so as to improve the high-temperature storage characteristics of the cell.
The ethylene carbonate content of less than 25% by volume, however, causes a problem of deteriorating the cycle characteristics. More specifically, when a non-aqueous electrolyte secondary cell is charged and discharged for several hundred cycles, unfavorable phenomena occur such as a decrease in cell capacity. This means that maintaining excellent cycle characteristics requires an ethylene carbonate content of not less than 25% by volume.
One known technique for improving high-temperature storage characteristics and cycle characteristics without changing the solvent composition is to add an appropriate additive to the electrolyte. Examples of this technique are shown in Patent Documents 2 to 5 below.
Patent Document 2 discloses a technique for improving the storage characteristics at 60° C. of a non-aqueous electrolyte secondary cell that is charged at high voltage by adding 1,3-dioxane to the electrolyte. This technique, however, causes a slight decrease in long-term cycle characteristics.
Patent Document 3 discloses a technique for improving the charge-discharge cycle characteristics of a non-aqueous electrolyte secondary cell and for reducing its swelling during storage by adding a dinitrile compound or the like to the electrolyte. This technique cannot fully improve the long-term cycle characteristics, either.
Patent Document 4 discloses a technique for improving the cell characteristics such as high-temperature cycle characteristics of a non-aqueous electrolyte secondary cell by adding a nitrile group-containing compound and a fluorotoluene compound to the electrolyte.
Additives can not only be added to an electrolyte, but also be mixed with an electrode active material as shown in Patent Document 5 below.
Patent Document 5 discloses a technique for achieving the thermal stability of a non-aqueous electrolyte secondary cell by either applying a nitrile compound such as aliphatic dinitrile to the electrode surface or adding the nitrile compound to the electrode active material. This document states, however, that adding a nitrile compound to an electrolyte increases the viscosity of the electrolyte, thereby decreasing the cell performance.    Patent Document 1: Japanese Patent Unexamined Publication No. 2005-317499    Patent Document 2: WO2007/139130    Patent Document 3: Japanese Patent Unexamined Publication No. 2008-108586    Patent Document 4: Japanese Translation of PCT Publication No. 2007-538365    Patent Document 5: Japanese Translation of PCT Publication No. 2007-519186